The present invention relates to the improvement of an illumination method with band and multiple wavelength rays when a double-focus optical apparatus utilizing chromatic aberration is employed in a position detector such as an X-ray exposure apparatus.
The inventor of the present application has already proposed "an illumination method with multiple and band wavelength rays in a double-focus detector utilizing chromatic aberration" in the Japanese Laid-Open Patent Application No. 2-816, which will be described briefly in the following.
In a stepper such as an X-ray exposure apparatus, it is required to arranged a mask and a wafer in position with a high accuracy. However, the surface of a wafer for use in a stepper is covered with a resist film or a transparent film, the thickness of which is not uniform, and therefore the intensity of reflected rays of an alignment mark on the wafer varies sinusoidally by the standing wave effect to light interference to cause variation in a relative intensity between signal rays for detecting the alignment mark. As a result, accuracy of alignment is remarkably reduced.
Accordingly, the aforesaid proposal has disclosed the following measure to solve such problem. In a double-focus detector utilizing a lens system having axial chromtic aberration in which a mask mark and a wafer mark are detected at the same time, as one focus plane of the lens system, an image forming plane of a single wavelength ray which corresponds to a first body, for example, which is the mask is employed and as the other focus plane of the lens system, an image forming plane of a given band wavelength ray or multiple wavelength rays which correspond to a second body, for example, in which the wafer is employed. With such arrangement, the first body is illuminated by a single ray of 500 nm or less and second body is illuminated by two wavelength rays of 500 nm or more, or of a given band wavelength flux so that a reduction in an alignment accuracy which is caused by light interference due to unevenness in thickness of a resist film or the like can be prevented.
In other words, in the prior art, the detection is made such that respective rays for illumination the mask and the wafer are selected to use two kinds of rays a single ray and multiple rays of flux in a given band of wavelengths are on both sides of the 500 nm wavelength. Namely, in an X-ray mask having a simple structure, the detection is made by illuminating with a single ray of 500 nm or less and in the wafer to which a process treatment is applied, the detection is made simultaneously by illuminating with multiple rays or a flux band with wavelengths flux of 500 nm or more. The reason is that rays of 500 nm or more are adapted to the process treatment of the wafer. On the other hand, the X-ray mask is formed with a single thin film or membrane of uniform thickness and is not influenced in the least by the process as compared with the wafer, so that a ray of 500 nm or less can be sufficientty employed.
By the way, the X-ray mask is a self-supporting strained film (membrane) which is good for transmitting X-rays for exposure and visible rays or infrared rays for alignment. However, an experiment conducted by the present inventor indicates that this membrane has transmissibility for visible rays which depends upon the wavelength of a ray and when a mark on the X-ray mask is detected with a single wavelength ray, for example, a g ray of 436 nm, there are some cases where a sufficient contrast of an image is not obtained because of the low transmissibility of the g ray. FIG. 10 shows a spectral transmissibility when an SiN film of 2 .mu.m thickness is employed as a membrane.
In FIG. 10, when the foregoing two kinds of ray are selected as rays for aligning the X-ray mask and the wafer, a spectral transmissibility of a ray to be used for detecting the X-ray mask on the short-wavelength side is about 0.1 and when such a ray is used for detection, a contrast of an image is largely reduced. Consequently, accuracy of detecting the position of the X-ray mask is significantly reduced, with the result that accuracy of detecting a relative position between the X-ray mask and the wafer is remarkably reduced.
To solve this problem, the following two methods are conceivable.
1 A single ray of 500 nm or more having a high spectral transmissibility to the X-ray mask is employed as an alignment ray to illuminate the X-ray mask. PA0 2 A sufficient image contrast is obtained even with a ray of 500 nm or less whose spectral transmissibility is low, by controlling thickness of a membrane film of X-ray mask.
The method 2 requires controlling a thichness of a membrane film which is made of Si N and is 1 .mu.m to 6 .mu.m thick. This is costly and results in a variety of restrictions in manufacturing. Consequently, the present invention adopts method 1, which is much better than method 2 in reducing restrictions on the whole system and cost of manufacturing the X-ray mask.